We report an investigation of electronic and optical properties of two-subband occupied electron gas confined in Al0.48In0.52As/Ga0.47In0.53As quantum well by far-infrared (FIR) modulated photoluminescence (PL) measurements. We show that, in addition to the characterization of the effective mass and Fermi energy, this technique is a useful tool for the understanding of the luminescence processes. Well-resolved Landau-level splittings were observed in the FIR modulated PL spectrum at a fixed magnetic field. It is found that the PL spectrum involves the transitions between localized holes and electrons at all occupied k states. For the FIR modulated spectrum monitored at the peak position as a function of magnetic field, a strong double cyclotron-resonance signal was observed. The double structure of cyclotron resonance is attributed to the cyclotron transitions in the first and second subbands. The obtained effective masses are found to be considerably heavier than the bulk value by 22% and 46% for the first and second subband, respectively. The heavier effective mass of the second subband is opposite to the behavior measured in the triangular quantum well. We point out that, in order to explain the enhanced masses, both the effects of band nonparabolicity and wavefunction penetration into the barrier material have to be taken into account. © 1998 American Institute of Physics.